I know this is a old thread. but my 2 c
I did not think of this until reciently. But I guess my wife and I are too fat to drive. Physically we both can. My wife does have rather difficult time getting behind the wheel, I do like watching. I can manage, my belly does hit the wheel.
The issues is the limit of the car, my wife is close to 550. I am up to 480. I was told the cars limit is 900. We barely fit ..

The stated payload of your car is not a problem. There's a huge safety margin factored in. What is a problem is a person's ability to safely operate a car. If a small child (or even a neighbors pet) runs out in front of your car and you can't quickly slam on the brakes because your smushed around the steering wheel or your legs are just too heavy to move quickly how are you going to feel.

People with physical limitations (of any type) have to be honest with themselves. If you're not physically able to drive safely hang up the keys.